Inkjet printing mechanisms use cartridges, often called "pens," which eject drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, ejecting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is supported by the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which substantially seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. The wiping action is usually achieved through relative motion of the printhead and wiper, for instance by moving the printhead across the wiper, by moving the wiper across the printhead, or by moving both the printhead and the wiper.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper, as well as on recently developed specialty coated papers, transparencies, fabric and other media.
As the inkjet industry investigates new printhead designs, the tendency is toward using permanent or semi-permanent printheads in what is known in the industry as an "off-axis" printer. In an off-axis system, the printheads carry only a small ink supply across the printzone, with this supply being replenished through tubing that delivers ink from an "off-axis" stationary reservoir placed at a remote stationary location within the printer. Since these permanent or semi-permanent printheads carry only a small ink supply, they may be physically more narrow than their predecessors, the replaceable cartridges. Narrower printheads lead to a narrower printing mechanism, which has a smaller "footprint," so less desktop space is needed to house the printing mechanism during use. Narrower printheads are usually smaller and lighter, so smaller carriages, bearings, and drive motors may be used, leading to a more economical printing unit for consumers. There are a variety of advantages associated with these off-axis printing systems, but the permanent or semi-permanent nature of the printheads requires special considerations for servicing.
The caps in these earlier service station mechanisms typically included an elastomeric sealing lip supported by a movable platform or sled. Typically, provisions were made for venting the sealing cavity as the cap lips are brought into contact with the printhead. Without a venting feature, air could be forced into the printhead nozzles during capping, which could deprime the nozzles. A variety of capillary passageway venting schemes are known to those skilled in the art, such as those shown in U.S. Pat. Nos. 5,027,134; 5,216,449; and 5,517,220, all assigned to the present assignee, the Hewlett-Packard Company. In the past, a separate vent path was used for each individual cap, often including a separate vent plug for each cap, which contributed to increasing the total part count for a printing mechanism, resulting in a more costly product in terms of both material and labor costs. Another vent system, first sold by the Hewlett-Packard Company in the DeskJet.RTM. 693C model color inkjet printer, provided a vent channel in a spring-biased cap base, over which an elastomeric cap was stretched into place.
For two-pen printers, earlier cap sleds were often produced using high temperature thermoplastic materials or thermoset plastic materials which allowed the elastomeric sealing lips to be onsert molded onto the sled. The elastomeric sealing lips were sometimes joined at their base to form a cup-like structure, whereas other cap lip designs projected upwardly from the sled, with the sled itself forming the bottom portion of the sealing cavity. Unfortunately, the systems which used a portion of the sled to define the sealing cavity often had leaks where the cap lips joined the sled. To seal these leaks at the lip/sled interface, higher capping forces were used to physically push the elastomeric lip into a tight seal with the sled. This solution was unfortunate because these higher capping forces may damage, unseat or misalign the printheads, or at the vary least require a more robust printhead design which is usually more costly. Moreover, while suitable for sealing two printheads using a single sled, the onsert molded designs were incapable of providing the wide deflection range required to use a single sled to seal more printheads, and in particular, four closely spaced printheads in an off-axis system.
A reliable capping system must accommodate for tolerance variations in the components of a printhead carriage, as well as variations in the fit of the pens when installed in the carriage. To properly align the pens for printing, each pen is constructed with a set of alignment datums which are tightly seated against a set of corresponding datums on the carriage. Even minor excursions from nominal values for these datums can impact the position of the printhead relative to the cap. Moreover, even if the datums are all within acceptable tolerance norms, occasionally a pen is not fully seated against a carriage datum, leading to tilted and/or twisted printhead orifice plates. A reliable capping system must be robust enough to adapt to these datum and pen seating variations.
Capping systems also need to provide an adequate seal while accommodating several different types of variations in individual printheads. For example, today's printhead orifice plates often have a waviness or ripple to their surface contour because commercially available orifice plates unfortunately are not perfectly planar. Besides waviness, these orifice plates may also be slightly bowed in a convex, concave or compound (both convex and concave) configuration. The waviness property may generate a height variation of up to 0.05-0.08 millimeters (2-3 mils; 0.002-0.003 inches). These orifice plates may also have some inherent surface roughness over which the cap must seal.
The typical way of coping with both the waviness problem and the surface roughness problem is through elastomer compliance, where a soft material is used for the cap lips. The soft cap lips compress and conform to seal over these irregularities in the orifice plate. Unfortunately, some printheads have widely varying maximum and minimum tolerances which mere elastomer compliance is unable to accommodate, so separate spring-biased gimballing cap sleds were required to seal each printhead, such as in the new off-axis style model 2000C inkjet printer produced by the Hewlett-Packard Company. These separate gimballing cap sleds increased the part count, as well as the labor time required to assemble the product, leading to more expensive printing mechanisms.
Rather than relying solely on elastomer compliance, where the elastomer is compressed to varying degrees during capping to ensure a tight seal, one earlier design used a suspended lip configuration, as shown in U.S. Pat. No. 5,448,270, assigned to the Hewlett-Packard Company, the present assignee. In this suspended lip design, a single sealing lip projected upwardly from a suspension-bridge like support. In this design, a hollow channel was formed along the underside of the cap to provide an air pocket down into which the "bridge" portion of the cap could be deflected when the lip required more compression to accommodate for manufacturing tolerance extremes than could be accommodated by mere elastomer compression.
In this suspended lip design, separate caps for each printhead were fit over four separate race or boss structures, sometimes referred to as a "chimneys," all formed on a single cap sled. Each boss served to locate the associated cap in position for sealing a printhead. Each boss had a central channel to provide additional room for the bridge portion to deflect downwardly for maximum desired deflection. Unfortunately, the separate caps required for each printhead further increased the part count for the unit, while also increasing the assembly costs because each cap had to be separately stretched over its boss on the sled. This stretching was required so in a relaxed state, the cap would resiliently grip the boss to provide the desired levels of diffusion resistance and venting. Moreover, because each cap is stretched and press-fit over its boss, cap-to-sled locating accuracy was more difficult to maintain than with onsert molded caps discussed above. The use of the boss to support the caps was believed to be a necessary component of the suspended lip design to adequately support the lip during sealing and ensure proper sealing forces, as well as to properly locate the lip around the printhead nozzles.
Proper capping requires providing an adequate hermetic seal without applying excessive force which may damage the delicate printheads or unseat the pens from their locating datums in the carriage. Moreover, it would be desirable to provide such a capping system which is easier to manufacture than earlier capping systems to provide consumers with a more economical, high quality inkjet printing mechanism.